Prime Minister Kevin Rudd says Australia condemns the Syrian regime for using chemical weapons on its own people.

Echoing comments by US president Barack Obama earlier today, Mr Rudd says the evidence is now overwhelming that the regime of Bashar al-Assad is behind last week's chemical attack in the suburbs of Damascus which killed hundreds of people in a "flagrant breach of international law".

"It is an offence against humanity and arguably is a crime against humanity," Mr Rudd told a news conference today.

"Therefore, it is now critical that the international community move towards agreement on a robust international response to the regime.

Key points

Obama: Syrian regime behind chemical attack

UK says it may act on Syria without UN mandate

UK says it wants to wait until inspection teams wrap up their work

UN team completes second sampling visit to Damascus

US officials say any strikes could last days

Russia, China, Iran warn of risks of wider war

Syrian government says rebels used chemical weapons

"In the absence of such a response, the problem is that this regime could then take succour that it could do this again.

"We do not believe that is the right course of action."

Defence Minister Stephen Smith says Australia may offer humanitarian assistance if needed and will provide "one of the leads" in the international community when it takes over the presidency of the Security Council on Sunday.

However, "no-one is looking to Australia for military assistance" and "no-one is contemplating troops on the ground" in Syria, Mr Smith says.

However, Mr Obama says he has not made a decision on whether to launch a military strike.

Mr Obama clearly signaled that expected US punitive strikes would be limited in scope, and would not be an attempt to tip the balance between Mr Assad's forces and rebels who have waged a bloody two-year civil war.

"I have not made a decision," Mr Obama told PBS NewsHour, when asked how close he was to signing off on US military action against Syria.

However, Mr Obama said he had concluded that "direct military engagement, involvement in the civil war in Syria, would not help the situation on the ground".

He said that by the end of any engagement, the Syrian government "will have received a pretty strong signal that it better not do it again".

Syria's government, supported notably by its main arms supplier Russia, blames rebel "terrorists" for releasing the toxins with the help of the US, Britain and France, and has warned there will be a "graveyard of invaders" if it is attacked.

With so many groups adding their names to those opposing the current regime, take a look at some of the most prominent.

However, British MPs will be given a chance to vote on whether military action should be taken before or after UN weapons inspectors report back with any evidence.

The Security Council, of which Britain is a permanent member, should be immediately briefed as soon as the inspections are complete and then "every effort" should be made to secure a resolution from the Security Council backing military action, a motion put before parliament said.

"Before any direct British involvement in such action, a further vote of the House of Commons will take place," the statement read.

Mr Hague says there must be a response to the use of chemical weapons.

"It's time the UN Security Council shouldered its responsibilities on Syria, which for the last two-and-a-half years it has failed to do," he said.

"We're clear that if there isn't agreement at the United Nations, then we and other nations still have a responsibility on chemical weapons."

Weapons experts have completed a second field trip to rebel-held suburbs of the capital Damascus, looking for evidence.

The inspectors travelled to the eastern outskirts of Damascus, with Opposition activists posting a video on the internet showing them in two locations talking to men, women and children affected by the attack.

They also took blood samples from the arms of some patients.

Russia and probably China set to veto any UN backing

Britain pushed the other four veto-holding members of the Security Council at the New York meeting to authorise military action against Syria - a move certain to be blocked by Russia and, probably, China.

Government claims rebels launched gas attacks

Meanwhile, Syria has asked the UN to order chemical experts to investigate three rebel attacks in which it says Syrian troops "inhaled poisonous gas".

Syrian UN ambassador Bashar Ja'afari says he has written Mr Ban to ask the weapons inspection team that also "investigate three heinous incidents that took place in the countryside of Damascus on (August) 22, 24 and 25 where members of the Syrian army inhaled poisonous gas".

"The (inspection) team has the ability to investigate other incidents as needed," Mr Haq said, adding that the initial three incidents they were due to examine when they arrived in Syria would be investigated "in due course".